Adsorption of Nitrogen Oxides on TiO2 Surface as a Function of NO2 and N2O5 Fraction in the Gas Phase

Band edges positions prediction of the of Ag nanocluster-decorated titania surfaces and their relationship to NO and NO2 interaction from first-principles calculations

Metal nanoclusters deposited on oxides have been widely used in photocatalysis playing an important role in the design of model catalysts with applications in heterogeneous catalysis. In particular, we are interested in the potential activity of these cluster-supported systems for the removal of nitrogen oxides either by possible catalytic reduction and/or by their adsorption. In this work, using first-principles methods, we evaluate the main characteristics of Agn (n = 1-4) nanoclusters isolated and deposited on anatase TiO2(101) and rutile TiO2(110) surfaces. Our results indicate that they are preferably adsorbed on rutile surface. The different formation energy at each surface can be explained using a Bader charge analysis. Particularly for Ag4 the lowest formation energy is obtained for tetrahedral geometry while the isolated Ag4 geometry is planar. Small silver deposits placed superficially on titania surfaces modify its electronic structures and improve the conduction band edges positions for possible NO reduction. Band edges positions with respect to the vacuum potential have been studied. The comparison of the conduction band minimum with the reduction potentials of NO/N2O and N2O/N2 shows that they are higher, being Ag3 on rutile and Ag1, Ag2 and Ag4P on anatase better for NO reduction. To complete the analysis, the calculation of work function, energy gap, ionization energy and electron affinity are relevant since they allow the location of semiconductor band edges at point of zero charge. Finally, the adsorption of nitrogen oxides is studied where the NO2 adsorption is favored over NO.

Metal Oxides as Catalysts and Adsorbents in Ozone Oxidation of NO x

NO _x represents an important group of pollutants that are formed in fuel combustion. As these pollutants cause significant environmental problems, removal of NO _x from exhaust gases is necessary. The present study investigated the influence of metal oxide powders on the removal of NO _x by oxidation with ozone. The aim of the study was to compare the catalytic effect of TiO₂, Al₂O₃, and Fe₂O₃ and to investigate the dependence of the effective rate constant on temperature. NO (400 ppm) and a variable concentration of ozone in a mixture of N₂ and O₂ was directed through the catalyst chamber and heated to 60-140 °C. The addition of metal oxides resulted in a significant increase in the efficiency of the oxidation of NO to N₂O₅. Fe₂O₃ had the largest effect with a maximum of an approximately 3-fold increase in the effective rate constant at 100 °C. At the same time Fe₂O₃ had the lowest NO _x adsorption capacity. In the case of all metal oxides, oxidation of NO to N₂O₅ caused an abrupt increase in adsorption of NO _x.